1. Field of the Invention
This invention resides in the fields of zinc electrowinning and particle bed electrolysis.
2. Description of the Prior Art
The final step in the production of zinc, following roasting and leaching, is electrowinning, in which aqueous zinc ion is electrolyzed to zinc metal at the cathode of an electrolytic cell while oxygen is liberated at the anode. One of the more efficient ways of electrowinning is the use of a particle bed electrode, and particularly a bed known as a "moving" or "moving packed" bed, as opposed to a stationary bed or a fluidized particle bed. In a moving bed, particle movement is maintained at a level that is high enough to prevent particle agglomeration, yet low enough to keep void space to a minimum and to keep at least most of the particles in contact with the current feeder. A disclosure of moving bed electrolysis is found in co-pending U.S. patent application Ser. No. 08/521,021, filed Aug. 30, 1995, now U.S. Pat. No. 5,635,051, entitled "Intense Yet Energy-Efficient Process for Electrowinning of Zinc in Mobile Particle Beds" (Salas-Morales et al.).
While the test data reported by Salas-Morales et al. was obtained with a sulfuric acid catholyte, the recovery of zinc from certain sources would benefit from an electrowinning process in which an alkaline catholyte is used rather than an acidic catholyte. Zinc-air batteries, for example, use an alkaline electrolyte, and an alkaline electrowinning cell would regenerate both the zinc particles and the alkaline electrolyte. Another example is the degalvanizing of (i.e., removal of zinc from) galvanized steel scrap, which is done by electrolytically aided leaching with hot caustic. This generates an alkaline zinc solution, from which zinc could be recovered directly if an alkaline electrowinning process were used.
Alkaline electrowinning of zinc with plate electrodes rather than particle bed electrodes results in the formation of a zinc powder rather than a layer of zinc on the cathode. This is because deposition of zinc from an alkaline solution results in the formation of dendrites on the cathode. The dendrites do not adhere well to the cathode and readily fall off in powder form. Neither agglomeration nor particle growth occur in the liberated powder, since deposition occurs on the cathode rather than on the powder. Use of this process was described by Dudek, F. J., et al. "Recycling Zinc by Dezincing Steel Scrap," An International Symposium on the Extraction and Applications of Zinc and Lead, Sendai, Japan, May, 1995. Zinc metal formed in this manner is difficult to wash and to melt because of its high surface area, and has a tendency toward dust formation.
Zinc deposition from an alkaline solution by use of a moving particle bed as the cathode is disclosed by Scott et al. in U.S. Pat. No. 4,272,333, issued Jun. 9, 1981. Scott et al. describe the depositions of copper, zinc, cobalt and manganese from various alkali, acidic and neutral solutions. The patent reports high current efficiencies (the amount of current used in reduction of the metal as a percentage of the total current consumed in the cell) together with energy consumption values that vary with the metal being deposited and the type of catholyte. In its data pertaining to zinc with an alkaline electrolyte, however, the patent reports current efficiencies of only 82-95%, and these efficiencies appear to drop as the initial zinc ion concentration increases. The energy consumption reported in these tests ranges from 2.5 to 3.4 kWh/kg, which is too high a range to be economically feasible, particularly when the recovered zinc is used in zinc-air batteries.
A moving bed electrowinning process conducted in an alkali electrolyte is therefore needed that will achieve high current efficiencies and achieve effective results with a low energy consumption. This and other limitations of the prior art are addressed by the present invention.